1. Field of the Invention
The invention relates to counter rotating aircraft gas turbine engines with counter rotating fans driven by counter rotating low pressure turbine rotors and, particularly, for such engines incorporating vanes to effect unequal power splits and variable torque between the counter rotating low pressure turbine rotors.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some low pressure turbines have been designed with counter rotating turbines that power counter rotating fans and booster or low pressure compressors. U.S. Pat. Nos. 4,860,537, 5,307,622, and 4,790,133 disclose counter rotating turbines with counter rotating rotors that power counter rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. Blade rows or stages of one of the counter rotating turbines, turbine rotor are interdigitated with blade rows or stages of another of the counter rotating turbine rotors. No vanes are disposed between the interdigitated rows of blades. A radially outer drum supports blade rows of one of the counter rotating turbines. These blade rows depend radially inwardly from the drum.
Advanced gas turbine commercial engines having counter rotating forward and aft fans and counter rotating boosters are being designed. It is desirable to design a counter rotating engine with a peak performance. It has been found that a peak performance can be attained when the front fan operates at a higher fan pressure ratio and higher rotational speed than the back fan. This can result in a substantial mis-match in horsepower and rotational speed between the counter rotating rotors. The counter rotating low pressure turbine is required to supply the necessary power to each of the forward and aft fans at the rotational speed of each fan. A conventional counter rotating turbine will operate at peak efficiency when the power split between both shafts is equal and when the rotational speeds are equal and opposite. In such a case, speed and horsepower ratios of the two rotors and turbines are substantially 1. It is highly desirable to have a gas turbine engine with counter rotating low pressure turbines that have different speed and horsepower ratios such as speed and horsepower ratios of 1.2 or more to attain peak fan efficiency.
An aircraft gas turbine engine includes a high pressure spool having a high pressure turbine drivingly connected to a high pressure compressor by a high pressure shaft and rotatable about an engine centerline. The gas turbine engine includes counter rotatable low pressure inner and outer spools. A low pressure turbine having a low pressure turbine flowpath is located aft of the high pressure spool. The low pressure turbine includes counter rotatable low pressure inner and outer shaft turbine rotors having low pressure inner and outer shafts, respectively, which are at least in part rotatably disposed co-axial with and radially inwardly of the high pressure spool. The low pressure inner shaft turbine rotor includes first low pressure turbine blade rows disposed across the low pressure turbine flowpath and drivingly connected to a first fan blade row by the low pressure inner shaft. The low pressure outer shaft turbine rotor includes second low pressure turbine blade rows disposed across the low pressure turbine flowpath and drivingly connected to a second fan blade row by the low pressure outer shaft. The first and second fan blade rows are disposed within a bypass duct radially outwardly bounded by a fan casing.
In some embodiments, at least some of the first low pressure turbine blade rows are interdigitated with some of the second low pressure turbine blade rows. At least one row of low pressure variable vanes is disposed between one interdigitated pair of the first and second low pressure turbine blade rows across the low pressure turbine flowpath. In other embodiments, the first low pressure turbine blade rows are not interdigitated with the second low pressure turbine blade rows. The counter rotatable low pressure inner and outer shaft turbine rotors are arranged in tandem with the inner shaft turbine rotor located aft of the outer shaft turbine rotor and the row of low pressure variable vanes disposed between the low pressure inner and outer shaft turbine rotors.
In the exemplary embodiment of the invention, at least one booster is drivingly connected to one of the low pressure inner and outer shafts and axially located between the first fan blade row and the high pressure spool. A low pressure turbine nozzle is disposed axially forward, upstream of, and adjacent to the first low pressure turbine blade rows.